Get a vocal coach. You need early guidance. You may show singing talent early on, and may even be quite good for your age, but you don't have the experience or knowledge it takes to develop on your own. A vocal coach will teach proper singing technique from the start, including how to sing from the diaphragm to avoid damaging the throat and how to control breathing to achieve greater vocal control. A vocal coach will provide exercises designed to strengthen your voice and teach specific vocal techniques that could take years to develop on your own. A good vocal coach will also encourage you and help foster your development by constantly challenging you.
Practice singing every day. Your voice is an untrained muscle. Vocal cords are changing daily. You should always seek to keep up with the development of your voice and exercise it like you exercise any muscle. The constant changing of your voice at this time will offer special challenges unique to you.
Sing in the local church choir and in your school choir. This will give you the opportunity to use your voice with other singers and will teach you how to harmonize. The instruction and encouragement you get from a school music teacher can also be helpful and inspire you to excel. School choirs often perform at concert events, which is a good way for you to build the self confidence necessary to sing well.
Enter contests. Start with local contests and work your way up to national competitions. Singing in contests allows you to sing in front of professional judges who often offer you valuable advice. Contests also present you with the opportunity to sing in front of audiences, and many contests offer prizes that could result in a chance for you to begin a professional career early on. With determination, practice and a supportive, nurturing environment, you will eventually develop into a skilled professional.